Traffic calming measures on the M42 which cost £40 million could be redundant if plans for a new motorway service station in Solihull are given the go-ahead, safety experts and campaigners claimed.
Concerns that congestion will return to a once-gridlocked stretch of motorway were raised on the eve of a public inquiry into plans for the station.
A Government inspector will consider two separate proposals for services along the M42 - one between Junctions 5 (Solihull) and 6 (NEC), close to the village of Catherine-de-Barnes, and a second at Junction 4 (Shirley).
Campaigners have heavily criticised plans to build on the green belt a complex which is two-and-a-half times the size of Catherine-de-Barnes.
They also claim building another junction in an already overcrowded part of the motorway network could put lives at risk.
Kevin Clinton, head of road safety at Birmingham-based Royal Society for the Prevention of Accidents (RoSPA), said the plans could jeopardise the massive improvements made in terms of combating congestion on the M42.
A series of speed cameras and electronic road signs erected on gantries between junctions 3a (M40) and 7 (M6) on the M42 has helped ease traffic congestion on a stretch of road that once had a reputation for gridlock.
The Active Traffic Management (ATM) system will now be rolled out to other parts of the country after it was branded a success by the Government.
Mr Clinton said: "The ATM seems to be working very well in terms of reducing congestion and moving traffic through that part of the motorway system.
"But building a new service station on the motorway could create problems that the ATM had sorted out.
"It has worked because it has helped to stop traffic speeding up and slowing down, but that will increase if there are cars going into and coming out of a new junction."
"The hard shoulder is used in this part of the motorway network to help traffic coming off junctions, but it makes it more complicated when you have a service station there."
The first public inquiry, which began in 1999, ended with the Secretary of State announcing that he was "minded to approve" the Catherine-de-Barnes application.
But the final go-ahead for development, which would swallow an ancient woodland, a Grade II listed farmhouse and 65 acres of green belt in the Meriden Gap, was never given.
A second public inquiry will begin tomorrow. after it was delayed from last June. The issue will be considered by a Government inspector over the next 25 days at Solihull Civic Suite.
Maggie Throup, the prospective Tory candidate for Solihull who set up Solihull Against Motorway Service Area Group, said road safety fears were a concern for residents.
Ms Throup said: "ATM and the service station are not compatible. The ATM system was set up to save lives but if the motorway becomes too complicated, and congested, then lives will be taken."
Lorely Burt, Liberal Democrat MP for Solihull, said: "The ATM system will all be for nothing if the service station is given the goahead because the M42 east of Solihull will become a car park.
"Birmingham International Airport and the National Exhibition Centre are conducting expansion plans so I cannot imagine why they would want to add to the congestion by building a service station in that area."
Caroline Spelman, Conservative MP for Meriden, said: "I just cannot understand why they are persisting with these plans.